In The Evil Dead franchise, Deadites are the bodies of human beings who have been possessed and then animated by demonic energies or by evil spirits, prolonged possession of which transforming them into an undead monstrosity, although they can take momentary normal human forms.
The anatomy of the possessed person alters slightly after becoming possessed; the teeth grow sharper, eyes turn pale, and skin decays slightly, possibly symptoms of the possession and deterioration of the body the entity is animating. The longer someone remains possessed by a demonic spirit, the more hideous they are in appearance. It is never revealed whether the Deadites in The Cabin in the Woods function in this way as well.
Summoning Item Edit
The Deadites are a direct reference to the Deadites of the Evil Dead series. It is possible the Deadites seen in this film were a hint specifically towards the 2013 Evil Dead remake/reboot/continuation.